1. Field of the Invention
The present invention relates generally to an apparatus for measuring fluid level and specifically to an apparatus for measuring fluid levels in hazardous environmental conditions.
2. Discussion of the prior art
Previous fluid sensing devices comprise a float generally mounted on the end of a lever arm whereby the float moves with the changing fluid level and the arm is connected to a potentiometer for providing a variable electrical output as the float moved along with the fluid level. However, such devices are not generally advantageous to use in a hazardous environment, for example, as a gasoline or diesel fluid level sensor. There is a possibility that there could be a small degree of arcing in the potentiometer or that a failure in the potentiometer itself could cause a spark igniting the fuel air mixture in the tank resulting in a possible explosion and fire.
In an attempt to avoid such difficulties, more recent fluid level sensing devices have utilized a float having a magnet mounted thereon located so as to move past a Hall effect device which provides an electrical indication when the fluid level is such that the magnet activates the Hall effect device.
Such systems had disadvantages in that in order to position the magnet relatively close to the Hall effect device, it is necessary to constrain movement of the float in a casing with the result that the float occasionally becomes jammed in the casing and fails to provide an accurate indication of the fluid level. If the clearances are made small enough so that jamming does not occur, then particles in the fluid or other debris can serve to jam the float inside the casing. Furthermore, in order to provide more than merely a single float level indication (the fuel level is less than or greater than the position marked by the Hall effect device) it is necessary to provide a plurality of Hall effect devices which essentially provide a digital fluid level readout.
In some instances the float may be mounted for pivotal movement rather than free floating in the measuring container. Whichever manner of mounting the float, it is always subject to a certain amount of sticking friction (stiction) and hysteresis in the pivot arm. This affects the accuracy of the device and requires that it normally be large enough so that the float moves through the full range of fluid level in order to provide an accurate output of the fuel level sensor. Furthermore, in order to change the measurement range, it is generally necessary to change the length of travel of the float so as to provide a measurement output over the desired increased length. This generally requires a redesign lengthening the pivot arm or the container constraining the float (in the case of a free floating fluid level sensor) which are major design changes.
Further, the digital output device requires multiple Hall effect devices and a suitable electronic circuit for scanning in some sequential fashion the individual Hall effect devices to determine the position of the float mounted magnet. Generally the magnet is located along the side of the float so that it passes adjacent the vertically oriented series of Hall devices. Such a magnet mounting allows for contamination of the magnet and the Hall devices by ferrous particles contained in the fluid flowing around the float.